Chemical process operations require enormous amounts of water. In the United States alone, more than 10 billion gallons of water are used every day for manufacturing processes. Accompanying this level of use, large volumes of impure water are produced. There is increasing concern over both the availability of water for chemical process operations and the potential hazards posed by contamination of the surrounding water environment by impure water.
Soybean refinery wastewater streams typically contain various impurities, including but not limited to BODs; CODs; vegetable oils, fats, and greases; processing reagents; miscellaneous organic salts; and suspended solids. Conventional soybean refineries typically generate 100,000 gallons per day of wastewater, including 1200 pounds per day of BODs, 2000 pounds per day of CODs, 290 pounds per day of vegetable oils, fats, and greases, and 180 ppm suspended solids. Discharge of such wastewater streams into municipal sewers or nearby rivers is economically inefficient and subject to strict state and federal environmental protection regulations. Ideally, such wastewater streams are treated so as to remove pollutants and produce process water suitable for reuse in the plant.
Treatment steps for removing wastewater impurities include clarification, coagulation, filtration, membrane separation, ion exchange, ultrafiltration, and chemical treatment. On one hand, by using tailored combinations of these options, contaminant levels can be routinely reduced to the parts per million (ppm) or parts per billion (ppb) range. On the other hand, conducting multi-step treatment can be complex and often requires costly equipment.
Perhaps the most common method of water treatment is the use of iron salt/alkaline flocculation techniques. Treatment with iron salt significantly reduces solubility of contaminants in the wastewater stream, leading to precipitation. However, due to the nature of the precipitate thus formed, which tends to be light, slow to settle, and very fine, treatment with iron salt alone generally has been believed to be inadequate to precipitate contaminants to a degree which permits collection and removal of contaminants to environmentally acceptable levels. For this reason, prior art methods generally add alkaline material in an amount sufficient to increase the pH of the wastewater stream to above about 9 in an effort to promote formation of ferrous hydroxide and/or ferric hydroxide. Alkaline addition is believed to enhance contaminant removal by inducing the precipitates, the contaminants, and/or the ferrous ions to form substantially insoluble coprecipitates with hydroxide ion.
For example, a prior method for removing heavy metal and/or radioactive contaminants from a wastewater stream is disclosed in U.S. Pat. No. 5,330,658. This method comprises mixing wastewater with an aqueous ferrous sulfate solution, adding a hydroxide solution to the resulting mixture, treating the resultant mixture with a flocculent, and separating coprecipitates from the purified stream. Typically the ferrous sulfate solution comprises from about 10 to about 25 percent by weight ferrous sulfate, and the hydroxide solution comprises from about 10 to about 50 percent by weight sodium hydroxide. A disadvantage of this method is that caustic alkali hydroxide solutions must be employed. Such solutions are corrosive and dangerous to handle.
Another prior method for treating wastewater is disclosed in U.S. Pat. No. 4,200,526 comprising the steps of mixing a wastewater stream with an iron salt and an alkaline solution to maintain a pH above about 8, separating effluent from the precipitated solids, filtering the separated effluent, adjusting the pH of the filtered effluent to below about 6, and subjecting the pH adjusted filtered effluent to reverse osmosis. Typically, about 510 pounds of ferrous sulfate and about 510 pounds of lime are required to process about 300,000 gallons of raw wastewater. A disadvantage of this method is the addition of a reverse osmosis step in the overall treatment scheme, which adds processing complexity.
U.S. Pat. No. 3,959,129 describes a process for treating wastewater of the type typically produced in conventional container manufacturing installations. The process comprises the steps of mixing a wastewater stream with an iron salt and agitating the resulting mixture, mixing calcium hydroxide with the agitated mixture and again agitating the resulting mixture, and finally passing the resulting mixture over the cylindrical outer surface portions of a horizontally rotating drum of a vacuum filter assembly. Generally, up to 5000 ppm iron salt and up to 5000 ppm calcium hydroxide are added, with an agitation time of about one hour after each addition. The effluent is drawn by vacuum into the interior of the vacuum filter assembly, and a filter cake is deposited on the outer cylindrical portions thereof. A disadvantage of this method is that it requires vacuum equipment and requires continual monitoring and removal of filter cake.
Further improvements in purifying wastewater streams continue to be sought. Thus, there is a need for a simplified, easy-to-operate method of treating large volumes of solutions containing soluble and insoluble organic and inorganic contaminants that effectively segregates the contaminants to produce a clean effluent and concentrates the contaminated material into a manageable, low-volume, concentrated waste stream. There is a further need for a wastewater treatment method that requires a minimal amount of equipment and is economical and safe to operate.
The present invention relates to a process having advantages over those previously disclosed. In particular, this invention relates to a method for treating soybean refinery wastewater with an aqueous iron salt solution to produce purified water that contains less than about 10 ppm iron and has a transmittance rating of at least about 90 percent. Surprisingly and unexpectedly, no addition of alkaline material is required in the present invention. The present invention allows virtually the entire wastewater stream produced by a soybean refinery to be treated and reused, thereby creating substantial savings.